Chapter 3. Environmental Response I
The most basic interest of ecologists is how organisms interact with their environment. Inanimate objects do not normally "interact"—they are simply acted upon by forces in the environment. Organisms, however, cannot be totally passive within a chain of events and still sustain life. In a sense, life represents a reversal of the "normal" order of things in a universe where physical and chemical reactions proceed, on balance—and usually rather directly—towards lower energy states.
When a volcano erupts it lays down rocks on the surface of its slopes. These rocks are elevated in energy state from their previous existence as physical material deep within the earth. But on balance, in order to erupt, that material was first heated until fluid, and then tremendous amounts of energy expended (as pressure) to push the material (magma) up to spew as lava from the top of the mountain. Only a small portion of that energy remains on the volcano in the form of potential energy of rocks perched on the surface. That potential energy will eventually be released after forces of erosion loosen the rocks and they roll downslope or are ground to silt and deposited in the ocean deep.
In essence, the physical environment (as distinct from the biological environment) is composed of various forms of matter and energy. The physical environment is one of material acted upon by various forces (energy), resulting in a lower energy state of the material with a concomittant release or conversion of energy as (usually) heat or light.
- Read Laws of Thermodynamics (Links need not be pursued at this time)
- Read Chapter on entropy in the Wikibook, Systems Theory
- Read Entropy (You need not delve into the mathematics unless so inclined; this article goes beyond "Basic Ecology" requirements)
- Read Time's arrow (This article will give you insight on why the concept of entropy even matters; remember, ecology is concerned with events at the macroscopic level)
Living systems—organisms—respond in various ways to contacts with the physical forces of an ever-changing environment and interrelationships with other living organisms. The hereditary potentialities of an organism determine what it can do, but the environment determines what it actually does and to what degree (Greulach & Adams, 1962). This theme is important and you will see it repeated in the discussions that follow.
Organism responses can be described under four different categories:
- behavioural, and
- community relations.
Although these are more or less distinct categories and will be discussed in detail in this chapter, an organism's response to an environmental challenge seldom involves just one of these. At the very least, the growth or development of a morphological feature involves internal physiological processes occurring between the genetic blue-print in the cells and some final expression of an anatomical trait. A protective morphological trait like horns on a bull come into play through behavioral actions. The horns are not a threat to a predator until wielded in an effective manner.
The Physical Environment
Why are all species not evenly distributed over the earth? A number of abiotic (non-living) factors shape the ecological patterns we observe over the face of the earth. Those factors include: water, temperature, wind, salt concentration, pH and the fact that the world is round. The fact that the earth is round has a substantial impact on the distribution of solar radiation, precipitation and wind across the Earth's surface. It is also responsible for the major movements of oceans. All of these abiotic factors naturally play an important role in the distribution and abundance of organisms across the globe.
Solar irradiation affects temperatures on Earth. Most organisms are mesophiles that like temperatures between 20-40 degrees C, some organisms are sychrophiles that like temperatures below 20 degrees C (cold-living organisms), and some organisms are thermophiles that like temperatures above 40 degrees C (heat-living organisms). Seasonality is somewhat associated with temperature. Some of the effects of seasonality include- (1) hibernation due to temperature and scarce resources, (2) rainfall/precipitation that determines how well certain plants or trees grow, and (3) wind.
Another aspect that is important to the diversity of animals' location is the amount of oxygen that is present in the particular area. For example, some organisms are aerobes which need oxygen for survival, and some are anaerobes which require the absence of oxygen for survival . However, not all organisms require the same amount of oxygen. Some organisms are microaerophiles and require little oxygen in order to live, but those organisms still do require oxygen for survival although it may not be as much as other organisms.
Dissolved oxygen plays a major role in supporting life in bodies of water. The greater the amount of oxygen in a body of water the more organisms the water can support. Algae release oxygen into the water which is later consumed by organisms during respiration. This process is efficient in the summer when the algae can consume adequate amount of sunlight. When lakes freeze over in the winter the snow covered ice blocks the sunlight from the algae. This causes a drop in oxygen production in the body of water since the organisms continue to respire. Low dissolved oxygen can cause a problem with predator/prey interactions within an ecosystem. Different organisms have different tolerance to low oxygen. If the predator can survive more efficiently than the prey, the amount of prey will rapidly decline. This can have a major effect on an entire food web. An experiment of this was conducted in the Chesapeake Bay. []
Water also plays a major role in the diversification of organisms throughout the world. As previously mentioned, the amount of precipitation is vital for plants to grow, which can provide food (serve as a food source) to other organisms. The salt concentration (salinity) is an important aspect to look at when dealing with water because some organisms are halophiles which are organisms that require salt in order to survive. The currents of the water that makes up the oceans is also important. The gyres are the surface currents in the oceans. The gyres distribute heat from the equators towards the poles causing the polar waters to have less energy than the water at the equator. The other type of current is the deep water current that sinks to the sea floor and flows towards the equator causing the polar waters to have a higher salinity and a higher density.
Meteorology plays an important aspect to life on Earth as well. The Earth is divided into three main parts- (1) Hadley Cell which is characterized as having Northeast trade winds and is mostly desert landscaping because of the cool, dry air, (2) Ferrell Cell which is characterized as having Westerlie winds, and (3) Polar Cell which is at the North and South poles of the Earth. The weather and wind effects plant dispersal (pollen and seeds getting blown) and the weather and the wind also effect the habitat range from which organisms can live in.
The pH is also a factor that contributes to life. The pH of the ocean is approximately 8.2. A deviation from this pH is slowly occurring, known as ocean acidification. Over a period of 243 years the pH of the ocean has dropped 0.075 in its pH. This slow acidification is a primary concern for calcium carbonate organisms. Structures made of calcium, including skeletons, are vulnerable to dissolution at lower pHs. Soil pH can range from 4.5 to 8.0, depending on area and vegetation growth. The main macronutrients in soil for vegetation are calcium, phosphorus, nitrogen, potassium, magnesium,and sulfur. Most vegetation prefers neutral to slightly basic or alkaline pH level, because at this level the macronutrients are readily picked up. Rain water is slightly acidic, depending on the amount of carbonic acid present. The average pH for rain water is 5.7. Natural freshwaters, such as lakes and streams, are also slightly acidic with a pH from 5.5 to 7.7. Drastic alteration in pH levels can have devastating effects. One example is acid rain. Acid rain has been known to destroy everything it touches, from minerals, to plant life, to animal and human life. To learn more about acid rain visit [].
- Read Ocean Acidification
The effect of wind on biodiversity varies greatly from one region to the next. For example, in southeast Alaskan rainforests, wind shapes the forests and the extent at which forests are composed. [] While in other parts of the world wind driven damage from hurricanes and tornadoes affect the biodiversity by destroying trees and plants used for food and shelter. Storms are partially driven by wind currents, which in turn bring more moisture to certain areas of the planet. Wind also has a major effect on the amount of time it takes water to evaporate back into the atmosphere. A pond will hold water longer when no wind is present to intensify the drying effect. Wildfires that destroy habitat for plants and animals can be greatly manipulated by winds. Winds can spread fires quickly and over great distances to further disrupt biodiversity.
Another major effect of wind is erosion. Deserts are continuously being moved and reshaped by erosion and sandstorms caused by wind displacing sand from one terrirory to another. Wind erosion also displaces soil for plant growth by removing the most fertile top layer of soil from the earth. Soil erosion limits the growth of plants and the diversity of animals and insects that feed on these plants.
The Dust Bowl in the 1930s is an excellent example of wind destruction across the plains of North America.
The dispersal of plants is greatly effected by wind driven pollen and seed distribution. Birds that feed on small wind driven seeds from one area will excrete the seeds in another area and in turn the seed can end up in numerous areas after being blown to one or two areas. The distribution of pollen and the cross pollination on plants brings more opportunities for animals to feed and have shelter, increasing the chance for reproduction and biodiversity. With deforestation diversity is greatly reduced, but winds without the obstruction of trees allow seeds and spores to be spread over a greater distance. This will help regenerate lost habitat and increase biodiversity. []
The fact that the Earth is round leads to several of the aspects that have been previously mentioned in the above paragraphs. Since the Earth is round, solar irradiation is distributed unevenly across the Earth's surface, which simply means that more solar radiation hits at the equator than at the poles. The less solar irradiation you have, the less energy there is, causing less biomass (living organisms and less diversity). All forms of life require some quantity of solar radiation in order to survive, but the amount varies from organism to organism.
Why does the temperature vary so greatly from the tropics to the poles? It is not simply a matter of more incident irradiation hitting the equator. Rather more irradiation impinges on a smaller area near the equator when compared to the poles. Furthermore, because sunlight strikes the poles at an angle it must traverse more of the atmosphere before being absorbed by the earth. As a result of having to travel through more of the atmosphere more of the solar energy is deflected or reradiated back into space. How does this impact life on earth? Less solar irradiation per unit area means that temperatures are generally cooler towards the poles which limits the kinds of organisms that may inhabit these environments. More irradiation near the equator means more energy is added to the ecosystem which translates in more biomass.
New problems in global ecology include rising levels of atmospheric CO2 concentrations and other greenhouse gases, and the resultant potential changes in global climate patterns. General Circulation Models (GCMs) (see Global Climate Model) are complex computer models developed by atmospheric scientists to help predict how increasing greenhouse gas concentrations might influence global climate patterns on a large scale.
For example, elevated temperatures, which are in part due to raised concentrations of carbon dioxide, affect turtle hatchlings. The map turtle and red-eared slider turtle both have temperature-dependent sex determination. A higher temperature results in a female hatchling, whereas a lower temperature results in a male. Thus, raised temperature result in fewer male turtles.
Global warming has a great impact on biodiversity distribution and abundance. Processes and patterns of the climate influence behavioral and physiological response of organisms, their productivity, relative competitive abilities of species, birth, growth, and death rates, nutrient cycling, decomposition, net primary production, and community structure of populations.
The form that an organism takes—termed its life-form—encompasses all of the structural aspects that make that species describable and unique. By morphological adaptations is meant the structural feature or features (called traits) that allow an organism to live successfully in its habitat; traits that have evolved over a (usually long) period of time as a consequence of the process of natural selection.
Natural Selection is a process that occurs over several generations. It allows select organisms to adapt to their environment over time. Natural selection often results in a species being able to survive in areas with environmental restrictions. For example, a plant that can grow taller than the others around it can get more sun light then the other plants around it and will increase its chance of survival and its overall fitness. For Natural Selection to occur, two requirements must be met. First there must be variation in some trait and second the trait must affect either survival or reproduction or both. Natural selection can result in Directional Selection or Disruptive selection. Directional Selection is when individuals at one end of the distribution do well and the other doesn’t. Disruptive selection is when there is an intermediate that does not do as well as the individuals on the other sides of the distribution.
- Read Wikipedia:en:Directional selection (for further information)
- Read Wikipedia:en:Disruptive selection
- Read natural selection (Links need not be pursued at this time)
Although we might conclude that adaptive morphological traits are acquired by a species slowly through the evolutionary process and therefore not subject to change within the lifespan of an individual, in fact many morphological traits are expressed or modified in response to environmental conditions. The individual organism inherits morphological variation within a range, and the expression of the trait represents a response to specifics of that individual's environment. The considerable similarity of all individuals of a species attests to the genetic basis of morphological traits, yet a degree of plasticity is usually present to be influenced by environmental factors. Individuals inherit this plasticity and not the growth-form of the parents. Thus growth-form represents the outcome of both "nature" (genetic or inherited traits) and "nurture" (influences of environment on the growth process).
A growth-form that persists through many generations because of environmental constancy (but is not an inherited trait) is termed an ecad. However, any such trait is subject to natural selection and may, over time, become fixed in a population as an inherited eco-type. That is, if the trait is highly adaptive within the habitat of the population, and the plasticity of expression is under-utilized becoming a genetic burden, that plasticity may be lost or restricted. Eco-types are genetically based differences between individuals of the same species that come from different populations.
Physiology is the study of biochemical functions of living organisms. Whereas morphology (discussed above) is essentially about anatomy and structure, physiology is largely about processes. However, the two are so interlinked in any organism that one cannot really be discussed without consideration of the other. As already noted, physiological processes must proceed the growth of morphological traits. And a physiological response, such as an increase in blood pressure, results from short-term mechanical changes in the anatomy of the heart and arteries. Put another way, the shape and structure of a heart is a morphological trait; the pumping of blood—the function of the heart organ—is a physiological activity. Muscle contractions are mechanical actions resulting from biochemical signals and changes. One cannot occur without the other.
Biologists refer to physiological adjustment as a response to environmental change. By "adjustment" is meant a change in physiology or internal function that allows the organism to maintain life. An adjustment is limited by the physiological and morphological adaptations inherited by the organism. Just as we previously described "morphological variation" as essentially an adjustment to environment based upon inheritance of a range of possibility, so too is physiology controlled by both inherited (genetic) and environmental (external) factors. The important distinction here is between the terms "adjustment" and "adaptation", a distinction that can apply to both morphology and physiology.
- Adaptations are physiological processes or morphological traits inherited through genetics and subject to natural selection at the population level;
- Adjustments are activities and traits expressed within an inherited range and subject to individual modification within the life-span of the individual or even much shorter periods of time. The proper term here is acclimatization.
If I work out and buff my body to rival that of the current Governor of California, I am not adapting but adjusting or acclimatizing myself (considerably, comparing our growth-forms!). On the other hand, when a caterpillar changes into a pupa and then transforms itself into a butterfly, that is clearly an adaptation. The process (called metamorphosis) is an inherited one, and irreversible. Aging is a process that would appear to be largely irreversible (certainly death is), and apparently is an adaptation. Once reproductive success peaks and starts to decline, advantage for the species population goes towards eliminating older individuals (fortunately, some exceptions exist).
Behavioral responses to environmental factors can be observed as movements of one sort or another. We say that "behavior" is the way an organism "acts"—what activities it "does". Complex behaviors controlled by cognitive processes evolved from simple response behaviors, the understanding of which provides a basis for understanding how behavior confers survivability on an organism. Note that such responses are generally rapid (and mediated through nerves in higher animals) in organisms capable of locomotor activity, and generally much slower in organisms lacking means of motility. Thus, plants, although devoid of musculature, nonetheless respond to stimuli of various sorts—growing or turning towards a light, for example. Simple turning responses, either towards or away from a specific stimulus, are termed tropisms. Actual physical movement towards or away from a stimulus are called taxes. Similar behaviours called kineses involve physical movements that appear random in orientation or direction, but the intensity of activity is determined by the intensity of the stimulus.
The "home" of the biosphere is the planet earth, an accretion body orbiting a star. The physical realities of the planet, resulting from its size and location relative to that star called the "sun", determine the basic physical properties within which life developed and presently exists.
Organisms respond to changes and vagaries in the environment. Organisms, unlike non-living objects, survive because of the ability to respond. Responses take many forms, which we can categorize as morphological, physiological, or behavioral. However, most responses involve a combination of these types, and sorting of responses into specific types typically just means that one is the more obvious aspect of the response observed.
- Greulach, V. A. and J. E. Adams. 1962. Plants. An introduction to Modern Botany. John Wiley & Sons, Inc., New york and London. 557 pp.
- Fresh Water, Natural Composition of. 2007. Advameg Inc. http://www.waterencyclopedia.com/En-Ge/Fresh-Water-Natural-Composition-of.html.
- Etchberger, Cory R., Michael A. Ewert, John B. Phillips, and Craig E. Nelson. Carbon Dioxide Influences Environmental Sex Determination in Two Species of Turtles. Amphibia-Reptilia 23 (2002): 169-175.